Eight limousines out more than 40 vehicles which took students to an end-of-year day-time school prom in South Derbyshire have failed safety checks.
Random inspections were carried out after pupils were dropped off at John Port School in Etwall on Thursday.
Defects with tyres, steering and no seat-belts were found on five.
All eight of the limousines which failed the checks got prohibition notices - meaning they must be proved roadworthy before operating again.
The tests were carried out by South Derbyshire District Council, the Vehicle and Operator Services Agency and police officers.
Five vehicles were being investigated for not having the relevant operator's licence and one driver was reported to police for not driving in accordance with a licence.
Councillor John Bladen, of South Derbyshire District Council, said: "John Port School was happy to invite us to carry out the inspections to ensure the vehicles were fit for purpose.
"The aim of this operation was to allow the pupils to enjoy their prom while carrying out vital checks to ensure their safety."
A district council spokesman added the authority had ensured the students all had transport home after the event.